This invention relates to high temperature heat exchangers and particularly, although not necessarily exclusively, heat exchangers in which a fluidised bed provides one of the materials in heat exchange relation.
For gas-to-gas heat exchange at temperatures too high for metal constructions, it is known to use ceramic heat exchangers, because they are capable of operating at higher temperatures than are obtainable from metal constructions. The use of ceramic materials poses a number of difficulties however.
For example, ceramic constructions are bulky as compared with metal constructions. This is firstly due to the relatively poor thermal conductivity of ceramics as compared with metals, and also because they cannot match the high heat transfer coefficients of a metal construction, particularly when the fluid to be heated is a liquid or gas under pressure. The total tube surface area in a ceramic heat exchanger must be of the order of four times its metal equivalent for the same heat transfer rate.
The bulk of a ceramic construction is increased further because of the more complex sealing arrangements required for the tube ends in order to limit thermal expansion stresses on the ceramic tubes. The minimum spacing between tubes is limited because of this requirement and even using a compact arrangement as described in U.S. patent application Ser. No. 6/9769 the tubes cannot be pitched closer than 1.8 tube diameters.
Although there may be many instances where the user is not concerned by the bulk of the apparatus, this adds to the cost and itself imposes design difficulties. For a given heat exchange rate, the total volume could be increased by increasing the length and numbers of the tubes, but increasing length accentuates the problems of material weakness as already mentioned, and it is undesirable to employ a plan form that is markedly oblong if heat losses are to be minimised.
Further problems arise from the brittle nature of ceramic materials, and especially their weakness in tension as compared with metals. In a tubed heat exchanger, where it is desirable to employ thin-walled tubes for efficiency of heat transfer, particularly having regard to the poor thermal conductivity of ceramics as compared with metals, these inherent weaknesses of ceramic materials can be a serious limitation.
The weakness of ceramic materials is also a significant factor in the problems that arise when trying to make seals between ceramic tubes and the end walls of a heat exchange chamber because of the need to allow for relative thermal expansion in high-temperature operation without overstressing the material. These difficulties are accentuated if the seals have to be capable of withstanding relatively high pressure differentials. Because of the fragility of ceramic materials and their high operating temperatures, seals suitable for metal heat exchanger tubes cannot be adapted to ceramic tubes.
Metal-tubed heat exchangers are also already known for fluidised bed heating apparatus. Such apparatus has gained acceptance in application to compact boilers and shallow bed water heaters, because of its advantages in being able to provide high heat transfer rates and uniform heating. In known systems, heat is extracted from the fluidised bed by passing the fluid to be heated, e.g. water or steam, through metal tubes which are submerged in the bed. There would be distinct advantages from the application of ceramic constructions to fluidised bed systems. For example, if heating clean air to high temperatures it is possible to show by theoretical calculations that a fluidised bed at 900.degree. C. could give heat transfer rates equivalent to a heat input in the form of a hot gas stream at 1600.degree. C.
However, ceramic material constructions have not been adopted for fluidised bed heating apparatus for practical reasons, and in particular because all the problems indicated above that come with the use of such materials would be encountered in a particularly severe form. For example the ceramic tubes submerged in the bed would be subjected to random forces greater than those typically experienced in a gas-to-gas heat exchanger and such forces can generate considerable local pressures that may crack a brittle ceramic material.
The increased bulk of ceramic constructions is also a disadvantage which is particularly apparent in the fluidised bed apparatus where it is possible to achieve a very high intensity of heating that allows compact metal constructions to be produced. Even if this disadvantage is accepted and the output rating of a fluidised bed apparatus using ceramic tubes is increased by accommodating more tubes in a deeper bed, that requires an increase of the fluidising gas pressure, which produces other problems.
The present invention has a special application to such fluidised bed heat exchange apparatus, although it can be usefully applied to other high temperature applications, such as for gas-to-gas exchangers.